The invention relates to a system and method for making downhole measurements in an underground borehole. The borehole may be related to the exploration or production of hydrocarbon fluids, such as crude oil and/or natural gas.
Boreholes for the production of crude oil and/or natural gas from a subsurface formation are generally drilled using a rotatable drill string. The drill string typically comprises a series of interconnected drill pipe sections. A downhole end of the drill string may typically be provided with a Bottom Hole Assembly (BHA) including sections of heavier drill collar to provide weight on bit, measuring while drilling (MWD) equipment, and a drill bit at the downhole end thereof for crushing the formation. A drilling rig at surface for holding the drill string is provided with a drive system for rotating the drill string, typically including a top drive or other rotary table.
One of the key hurdles in achieving real time subsurface navigation lies in a communication bottleneck between surface and downhole. Currently available measurement while drilling tools and logging while drilling tools can measure all vital information downhole, except depth. Real time availability of a depth estimate downhole can open new possibilities for real time automated bit steering and optimization in well drilling operations.
For the navigation in subsurface, one of the most important measurement is borehole depth at any point in time. Along with directional measurements of azimuth and inclination it gives the location of the drill bit with respect to a surface location. There are several uses of the depth measurement, such as in locating geological features in the formation, in following an optimized borehole trajectory, for the calculation of casing shoe depth, for estimating cement quantities, etc.
In current system of depth measurement, a surface system may record the time and length of the drill string below the rig floor. The drill string length may be used as standard depth measurement. The Kelly bushing or rig floor may be used as reference for land based rigs and mean sea level for offshore rigs. The length of the drill string (combined lengths of the BHA and the drill pipe sections) to the top drive (or traveling block) and the position of the top drive (or traveling block) in the derrick is used to determine the depth of the drill bit and the rate of penetration (ROP).
The movement of the traveling block is measured by drilling line payout from the draw works, which is either calibrated with draw work rotation or measured with help of a geolograph. Offshore, a heave compensator may be used to eliminate effects of heave in floating offshore facilities. Despite all the efforts to make an accurate measurement of depth, current systems are prone to errors due to factors related to thermal expansion, drill pipe stretch, pressure effects, and/or an error in pipe tally, drill string sensor calibration and heave correction. The total error in depth due to these factors can be up to 10-12 m over 3000 m depth. There have been efforts to calculate the error related to the above mentioned phenomena and use this as a correction to determine correct depth but they do not quantify the error accurately and are seldom used in practice. Downhole measurement of depth has a potential to eliminate these errors.
U.S. Pat. No. 5,341,886 discloses a method and apparatus for controlling the direction of advance of a rotary drill. The system comprises a drill string, a rotatable drill bit carried on the drill string, a roratable bit, and a compliant subassembly facilitating changes in the direction of drilling. The system includes a magnetic marker assembly, comprising a formation magnetizer. Magnetic markers in the formation are created by corresponding current pulses through the magnetizer. The tool also includes magnetic sensors, spaced a distance L from the magnetizer. When the magnetic sensors detect a magnetic marker, the magnetizer is urged to create another magnetic marker. New magnetic markers are created each spaced a distance L apart, allowing measurement of incremental depth.
In practice, the system of U.S. Pat. No. 5,341,886 proves to have drawbacks rendering it unsuitable for application. The formation often cannot be magnetized, creating significant errors in the depth measurement. The disclosure of U.S. Pat. No. 5,341,886 indicates the necessity to use high intensity magnetic pulses, in the order of a few thousand oersteds at the pole faces. In addition to significant power requirements, the high intensity magnetic pulses may interfere with magnetisable materials in the tool string.
U.S. Pat. No. 7,283,910 discloses a method and apparatus for logging an earth formation and acquiring subsurface information to obtain parameters of interest, which may include density, porosity, acoustic reflectance, a nuclear magnetic resonance property, or electrical resistivity. The parameters are acquired with a plurality of sensors. Time separation values between signals from separate sensors are determined. Using known sensor spatial separations and time separation values, a drill rate is determined and an incremental depth for the subsurface feature is defined.
The apparatus of U.S. Pat. No. 7,283,910 emits nuclear energy, and more particularly gamma rays. A gamma ray source is combined with two or more gamma ray detectors, shielded from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. The sensor may include a nuclear magnetic resonance sensor.
While drilling, the two or more sensors pass the same location in the formation at a different time, depending on their separation and the rate of penetration. An algorithm can be used to compare the outputs from these sensors in time to correlate the character of the signals which come from the same subsurface formation. As these signals correspond to the same subsurface location, the progress along the borehole as well as the rate of penetration can be calculated, using the known distance L between the sensors. I.e., the drill string has progressed a distance L along the borehole during the time difference Δt=(t2−t1). The rate of penetration ROP=L/(t2−t1). By integrating the ROP, incremental depth can be calculated.
The apparatus of U.S. Pat. No. 7,283,910 has a few challenges. For instance, in horizontal sections, wherein the borehole typically extends within a formation layer and in the same lithology, the characteristic rock property signature might not change significantly enough to distinguish between the signature and noise. In addition, logs never repeat exactly. The minor variations in successive gamma ray measurements are usually statistical fluctuations due to the random nature of the radioactive pulses reaching the detector or sensor. For example, the accuracy of gamma ray tools is around 5% in general and precision is inversely proportional to the square root of the logging speed and will be affected by a change in instantaneous speed. Fluid invasion and change in hole diameter, for instance due to washouts, can also change the measured values over time. With the integration of ROP, an integration error is introduced in the calculated incremental depth, which error will increase with increasing depth. The size of the error depends, for instance, on ROP variation, distance between the sensors and frequency of measurements. Finally, the precision of logging tools decreases with increasing temperature, which limits high temperature application.
U.S. Pat. No. 7,999,220 discloses an assembly of a pulsed neutron source and a gamma ray detector for borehole logging. The detector assembly comprises a lanthanum bromide (LaBr3) scintillation crystal and a digital spectrometer that cooperates with the crystal and a digital spectrometer that cooperates with the crystal to maximize pulse processing throughput. The assembly is applicable to borehole logging methodology that uses the measure of gamma radiation in harsch horehole conditions. The system is particularly applicable to carbon/oxygen logging.
European patent application EP 2615477 discloses a neutron logging tool for measuring azimuthal distribution of proppant in formation fractures. The tool has a neutron source and multiple detectors spaced about the circumference of the tool. The detectors are shielded from each other such that each detector detects gamma rays from the area of the borehole and formation to which it is closest. To capture a log with the tool, the neutron source sends high energy neutrons into the surrounding formation. The neutrons quickly lose energy as the result of scattering, after which they are absorbed by the various atoms within the ambient environment. The scattered and absorbed neutrons emit gamma rays with characteristic energies. These gamma rays can be measured versus characteristic energy and the presence or absence of certain materials can be determined.
International patent application WO 2006/004740 discloses a downhole pulsed neutron emission and detection technique for determination of fluid flow velocity in a borehole.
US patent application US 2008/251710 discloses a wireline suspended borehole logging tool for determining silicon content of a formation equipped with a pulsed neutron emission source and nuclear radiation sensors and a processor located at the earth that may be configured to estimate the velocity of the logging tool based on the measurements made by the sensors transmitted to the surface processor via the wireline from which the logging tool is suspended. It would not be obvious for a skilled drilling expert who wished to estimate depth of a borehole being drilled to find a solution to this problem to consult US 2008/251710 relating to a wireline logging tool for determining silicon content in a formation. US 2008/251710 neither teaches that instead of transmitting tool velocity measurements made by downhole neutron emission source and nuclear radiation sensors to a processor at the earth surface these velocity measurements may be transmitted to a downhole tool, thereby obviating the need to transmit a large amount of data to a processor at the earth surface via a data transmission wireline, which is not practical or expensive to insert into a rotary drilling assembly.
Prior art documents provide wireline operated or relatively inaccurate borehole depth measurement tools. None thereof however can measure depth without requiring data transmission via a wireline to a surface processor and/or with an accuracy which would be sufficient for automated drilling.
Therefore there is a need to improve upon one or more of the systems and methods as described above.
Furthermore there is a need for an accurate downhole drilling progress monitoring unit, which can provide accurate real-time incremental depth of an automated or other drilling assembly during drilling without requiring complex and fragile wired or wireless data transmission equipment extending from a Bottom Hole Assembly (BHA) of the drilling assembly to the earth surface.